With IR cameras it is often desirable to present an enlarged partial image of the camera's normal field view on a display screen. In such cases it is known to provide the IR camera with an object lens having a variable magnification and image field, e.g. a double lens or zoom lens. Such object lenses give improved resolution in the partial image, i.e. this image has a greater degree of detail than the normal image. These object lenses are complicated, however, and it is difficult to meet heavy demands on exactness of the object lens sighting line, which has importance, e.g. in aiming for firing purposes. A fixed object lens is therefore often used in IR cameras in order to obtain sufficient accuracy in a simple manner. An enlarged partial image can thus be obtained conventionally by the video signal for a line in the field of view from one of the IR camera detector elements being presented on two mutually adjacent lines on the display screen. To a certain extent this makes it easier for the operator to identify an object, but the partial image has the same resolution as the normal image. It is also known to present a partial image in which each line on the display screen contains superimposed information from several scans of a line in the IR camera's field of view. The signal-to-noise relationship in the picture is thus improved, i.e. the noise interference in the partial image, coming from such as the electronic circuits in the camera will be less noticeable than in the normal image. Scanning the field of view takes a long time, however, and the resolution in the partial image is not improved. In order to obtain improved resolution in an IR camera it has been proposed to provide the detector with an increased number of detector elements. The detector is so complicated and expensive that this is hardly economically possible, however.